THE PROTON-SUCROSE SYMPORT

The heterotrophic tissues of the plant are dependent upon carbon and nitrogen import for normal growth and development. In general, oxidized forms of these essential elements are reductively assimilated in the leaf and, subsequently, sucrose and amino acids are transported to the heterotrophic cells in a process known as assimilate partitioning. In many plant species, a critical component of the assimilate partitioning pathway is the proton-sucrose symport. This active transport system couples sucrose translocation across the plasma membrane to the proton motive force generated by the H+-pumping ATPase. To date, the proton-sucrose symport is the only known system that can account for sucrose accumulation in the vascular tissue of the plant. This review focuses on recent advances describing the transport properties and bioenergetics of the proton-sucrose symport.